Long-Term Stability of Acoustic Benchmarks Deployed on Thick Sediment for GPS/Acoustic Seafloor Positioning - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 270

Fig. 7 A sample of a push core with good penetration. The size of the corer is 4.8 cm in diameter
and 30 cm in length. Some 5 cm of the topmost part of the sediment is quite soft and the color is
different from the lower part
The benchmarks were dropped from the sea surface, fell in the sea water, and
landed on the sediment. It sank partly into the sediment due to the shock of the
landing. Judging from the photographs shown above, the sediment was stiff enough
to stop the penetration within 10-20 cm. The average density of the instrument is
only slightly higher than the sea water in order to reduce the shock at the landing.
The density of the unconsolidated sediment is not so different from the sea water.
The instrument is designed to keep the attitude vertical during the free-fall in
the sea water. Therefore, there is little force for it to further sink into the sediment
or tilt.
Considering that the bottom part of each benchmark is partly buried in the sedi-
ment, the problem is not a horizontal movement but a tilt. Tilting of an instrument
on the seafloor may probably result from its sinking into the sediment. Stiffness of
the surface layer of the sediment can be estimated from the above-mentioned pen-
etration at the landing on the sediment. It was also estimated from push-coring. The
coring showed that the sediment was very soft in the top 4-5 cm, became gradually
stiff with the depth, and stopped push-coring at 15-25 cm penetration. Because the
size of the bottom part of a PXP is much larger than the diameter of the push-corer
(4.8 cm), attitude of each benchmark is stable on the sediment with a large resis-
tance against sinking.